Quite briefly, they are the serpent-people of India. The male are known as Nagas while the female are called Naginis. They resemble humans from the waist up, and snakes from the waist down. Sometimes they have multiple heads and vary in colour. Also, at times Nagas are depicted as limbless and having a cobra-like frill.
The four classes of Naga are as follows: Heavenly; Divine; Earthly and Hidden. All Nagas / Naginis are characterised by their various functions (guarding the heavenly palace; giving rainfall; draining rivers or guarding rivers - respectively). It is said that they will eventually destroy the world with fire.
In Burma, Nagas are part serpent, part dragon and part crocodile. They give rubies to those they favor and protect many royal people. In Indonesia, Thailand, and Malaysia, however, Nagas are giant black Water-Snakes or multiheaded Sea-Dragons that terrorize fishermen. Their images are often used as temple guardians.
The name has been applied to the Indian cobra, Naja naja.
See also Lamia